5MHC restriction of cytolytic T lymphocytesMHC restriction of cytolytic T lymphocytes. Virus-specific cytolytic T lymphocytes (CTLs) generated from virus-infected strain A mice kill only syngeneic (strain A) target cells infected with that virus. The CTLs do not kill uninfected strain A targets (which express self peptides but not viral peptides) or infected strain B targets (which express different MHC alleles than does strain A). By use of congenic mouse strains that differ only at class I MHC loci, it has been proved that recognition of antigen by CD8+ CTLs is self class I MHC restricted

7Antigen-presenting cells are required for T cell activationAntigen-presenting cells are required for T cell activation. Purified CD4+ T cells do not respond to a protein antigen by itself but do respond to the antigen in the presence of an antigen-presenting cell (APC). The function of the APCs is to present a peptide derived from the antigen to the T cell. APCs also express costimulators that are important for T cell activation; these are not shown.

9Functions of different antigen-presenting cellsFunctions of different antigen-presenting cells. The three major types of antigen-presenting cells for CD4+ T cells function to display antigens at different stages and in different types of immune responses. Note that effector T cells activate macrophages and B lymphocytes by production of cytokines and by expressing surface molecules; these will be described in later chapters.

23Antigen processing requires time and cellular metabolism and can be mimicked by in vitro proteolysis. If an antigen-presenting cell (APC) is allowed to process antigen and is then chemically fixed (rendered metabolically inert) 3 hours or more after antigen internalization, it is capable of presenting antigen to T cells (A). Antigen is not processed or presented if APCs are fixed less than 3 hours after antigen uptake (B). Fixed APCs bind and present proteolytic fragments of antigens to specific T cells (C). The artificial proteolysis therefore mimics physiologic antigen processing by APCs. Effective antigen presentation is assayed by measuring a T cell response, such as cytokine secretion. (Note that this type of experiment is done with populations of antigen-specific T cells, such as T cell hybridomas, which respond to processed antigens on fixed APCs, but that normal T cells require costimulators that may be destroyed by fixation. Also, the time required for antigen processing is 3 hours in this experiment, but it may be different with other antigens and APCs.)

25Pathways of antigen processing and presentationPathways of antigen processing and presentation. In the class II MHC pathway (top panel), extracellular protein antigens are endocytosed into vesicles, where the antigens are processed and the peptides bind to class II MHC molecules. In the class I MHC pathway (bottom panel), protein antigens in the cytosol are processed by proteasomes, and peptides are transported into the endoplasmic reticulum (ER), where they bind to class I MHC molecules. Details of these processing pathways are in Figures 5-10 and TAP, transporter associated with antigen processing.

26Presentation of extracellular and cytosolic antigensPresentation of extracellular and cytosolic antigens. When a model protein ovalbumin is added as an extracellular antigen to an antigen-presenting cell that expresses both class I and class II MHC molecules, ovalbumin-derived peptides are presented only in association with class II molecules (A). When ovalbumin is synthesized intracellularly as a result of transfection of its gene (B), or when it is introduced into the cytoplasm through membranes made leaky by osmotic shock (C), ovalbumin-derived peptides are presented in association with class I MHC molecules. The measured response of class II-restricted helper T cells is cytokine secretion, and the measured response of class I-restricted CTLs is killing of the antigen-presenting cells

27Presentation of extracellular and cytosolic antigensPresentation of extracellular and cytosolic antigens. When a model protein ovalbumin is added as an extracellular antigen to an antigen-presenting cell that expresses both class I and class II MHC molecules, ovalbumin-derived peptides are presented only in association with class II molecules (A). When ovalbumin is synthesized intracellularly as a result of transfection of its gene (B), or when it is introduced into the cytoplasm through membranes made leaky by osmotic shock (C), ovalbumin-derived peptides are presented in association with class I MHC molecules. The measured response of class II-restricted helper T cells is cytokine secretion, and the measured response of class I-restricted CTLs is killing of the antigen-presenting cells

28Presentation of extracellular and cytosolic antigensPresentation of extracellular and cytosolic antigens. When a model protein ovalbumin is added as an extracellular antigen to an antigen-presenting cell that expresses both class I and class II MHC molecules, ovalbumin-derived peptides are presented only in association with class II molecules (A). When ovalbumin is synthesized intracellularly as a result of transfection of its gene (B), or when it is introduced into the cytoplasm through membranes made leaky by osmotic shock (C), ovalbumin-derived peptides are presented in association with class I MHC molecules. The measured response of class II-restricted helper T cells is cytokine secretion, and the measured response of class I-restricted CTLs is killing of the antigen-presenting cells

29Presentation of extracellular and cytosolic antigensPresentation of extracellular and cytosolic antigens. When a model protein ovalbumin is added as an extracellular antigen to an antigen-presenting cell that expresses both class I and class II MHC molecules, ovalbumin-derived peptides are presented only in association with class II molecules (A). When ovalbumin is synthesized intracellularly as a result of transfection of its gene (B), or when it is introduced into the cytoplasm through membranes made leaky by osmotic shock (C), ovalbumin-derived peptides are presented in association with class I MHC molecules. The measured response of class II-restricted helper T cells is cytokine secretion, and the measured response of class I-restricted CTLs is killing of the antigen-presenting cells

39Role of TAP in class I MHC-associated antigen presentationRole of TAP in class I MHC-associated antigen presentation. In a cell line lacking functional TAP, class I molecules are not efficiently loaded with peptides and are degraded, mostly in the endoplasmic reticulum (ER). When a functional TAP gene is transfected into the cell line, normal assembly and expression of peptide-associated class I MHC molecules are restored. Note that the TAP dimer may be attached to class I molecules by a linker protein called tapasin, which is not shown in this and other illustrations. TAP, transporter associated with antigen processing.

46Presentation of extracellular and cytosolic antigens to different subsets of T cells. A. Extracellular antigens are presented by macrophages or B lymphocytes to CD4+ helper T lymphocytes, which activate the macrophages or B cells and eliminate the extracellular antigens.B. Cytosolic antigens are presented by nucleated cells to CD8+ CTLs, which kill (lyse) the antigen-expressing cells.

47Immunodominance of peptidesImmunodominance of peptides. Protein antigens are processed to generate multiple peptides; immunodominant peptides are the ones that bind best to the available class I and class II MHC molecules. The illustration shows an extracellular antigen generating a class II-binding peptide, but this also applies to peptides of cytosolic antigens that are presented by class I MHC molecules. APC, antigen-presenting cell.